Hi, Currently libvirt supports metadata in the domain XML. This is very convenient for data related to the VM, but it is a little awkward for devices. Let's pretend I want to have extradata (say, a specific port for a virtual switch) related to a device (say, a NIC). Nowadays I can store that data in the metadata section, but I need some kind of mapping to correlate this piece of information to the specific device. I can use the device alias, but this is not available when the device is created. This is also more complex when doing hotplug/hotunplug, because I need to do update device and update metadata; if either fails, the entire operation must be considered failed. It would be nice to be able to attach metadata to the device, and this is what I'm asking for/proposing in this mail. Would it be possible in a future libvirt release? If this is not possible, what's the best way to do the aforementioned mapping? If it's alias (or device address), how can I be sure than I'm addressing (no pun intended) the right device when I don't have them? (e.g. hotplug new device, or just first time VM created)? Thanks, -- Francesco Romani Red Hat Engineering Virtualization R & D IRC: fromani
Ping? Would it be feasible? Or is the very basic idea unfit? bests, On 02/27/2017 04:41 PM, Francesco Romani wrote:> Hi, > > > Currently libvirt supports metadata in the domain XML. This is very > convenient for data related to the VM, but it is a little awkward for > devices. Let's pretend I want to have extradata (say, a specific port > for a virtual switch) related to a device (say, a NIC). Nowadays I can > store that data in the metadata section, but I need some kind of mapping > to correlate this piece of information to the specific device. > > I can use the device alias, but this is not available when the device is > created. This is also more complex when doing hotplug/hotunplug, because > I need to do update device and update metadata; if either fails, the > entire operation must be considered failed. > > It would be nice to be able to attach metadata to the device, and this > is what I'm asking for/proposing in this mail. > > Would it be possible in a future libvirt release? > > > If this is not possible, what's the best way to do the aforementioned > mapping? If it's alias (or device address), how can I be sure than I'm > addressing (no pun intended) the right device when I don't have them? > (e.g. hotplug new device, or just first time VM created)? > > > Thanks, > >-- Francesco Romani Red Hat Engineering Virtualization R & D IRC: fromani
For techincal discussions on design it's better to post to the development list. On Mon, Feb 27, 2017 at 16:41:28 +0100, Francesco Romani wrote:> Hi,Hi,> > > Currently libvirt supports metadata in the domain XML. This is very > convenient for data related to the VM, but it is a little awkward for > devices. Let's pretend I want to have extradata (say, a specific port > for a virtual switch) related to a device (say, a NIC). Nowadays I can > store that data in the metadata section, but I need some kind of mapping > to correlate this piece of information to the specific device. > > I can use the device alias, but this is not available when the device is > created. This is also more complex when doing hotplug/hotunplug, because > I need to do update device and update metadata; if either fails, the > entire operation must be considered failed. > > It would be nice to be able to attach metadata to the device, and this > is what I'm asking for/proposing in this mail. > > Would it be possible in a future libvirt release?In general I don't have a problem with this and if it would make the life of mgmt tools easier I don't see a reason why not. There's one problem though. For setter (or getter) APIs for the metadata you need a way how to uniquely identify a device on which you want to operate. There's no such way currently in use in libvirt so you need to come up with something to do it. The identifier is also desired for the metadata change event which would go with the APIs. It might be possible to do this even without APIs but the usefulnes would be rather limited as it was with the domain metadata.
On Thu, Mar 16, 2017 at 01:46:38PM +0100, Peter Krempa wrote:> > For techincal discussions on design it's better to post to the > development list. > > On Mon, Feb 27, 2017 at 16:41:28 +0100, Francesco Romani wrote: > > Hi, > > Hi, > > > > > > > Currently libvirt supports metadata in the domain XML. This is very > > convenient for data related to the VM, but it is a little awkward for > > devices. Let's pretend I want to have extradata (say, a specific port > > for a virtual switch) related to a device (say, a NIC). Nowadays I can > > store that data in the metadata section, but I need some kind of mapping > > to correlate this piece of information to the specific device. > > > > I can use the device alias, but this is not available when the device is > > created. This is also more complex when doing hotplug/hotunplug, because > > I need to do update device and update metadata; if either fails, the > > entire operation must be considered failed. > > > > It would be nice to be able to attach metadata to the device, and this > > is what I'm asking for/proposing in this mail. > > > > Would it be possible in a future libvirt release? > > In general I don't have a problem with this and if it would make the > life of mgmt tools easier I don't see a reason why not. > > There's one problem though. For setter (or getter) APIs for the metadata > you need a way how to uniquely identify a device on which you want to > operate. There's no such way currently in use in libvirt so you need to > come up with something to do it.Once you have a reliable identifier for a device, it is not compelling to store metadata against each device as you can reliably identify the device from data in the existing metadata block. I don't find the atomicity argument very compelling, because even if hidden behind a single libvirt API, it is still fundamentally multiple operations in libvirt which are just as likely to fail as if the app did multiple libvirt API calls as we're not able to provide any kind of transactional guarantees here. IOW, we could effectively solve this by assigning the device alias names at XML define time, instead of VM launch time. We historically didn't do this since we wanted to cope with old QEMU which didn't allow custom device aliases. We now mandate a new QEMU that supports -device, so that's no longer a problem. So I'd prefer to see us just define device aliases persistently and let apps use those to track metadata in the global metadata section. Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://entangle-photo.org -o- http://search.cpan.org/~danberr/ :|